1. Field of the Invention
The present invention relates to a method for controlling the speed of a sled motor of an optical disk drive. More particularly, the present invention relates to a method for controlling the speed of a sled motor of an optical disk drive which uses a velocity profile to control the speed of the sled motor. The present disclosure is based upon Korean Application No. 96-6647, which is incorporated herein by reference.
2. Description of the Related Art
A compact disk (CD) player and a digital video disk (DVD) player are devices for reproducing audio data stored on an optical disk. On the other hand, a CD-ROM drive and a DVD-ROM drive are computer peripheral devices for reproducing digital data stored on an optical disk. The CD-ROM drive and DVD-ROM drive are random access devices, unlike the CD player and the DVD player, and thus make frequent movements to read the data on the disk.
In such an optical disk system in which a rotating speed and a pick-up position need to be accurately controlled while simultaneously maintaining a high speed operation, such as in the CD-ROM drive, system performance is maximized when the sled and spindle servos of the device are accurately controlled and are in appropriate harmony with one another.
FIGS. 1 and 2 show the sled driving shaft 14 and its peripheral devices in the CD-ROM drive. More specifically, FIG. 1 shows the structure of a typical transferring mechanism of a pickup device in a CD-ROM drive, and FIG. 2 shows the relative position of a sled motor and Hall-effect sensor in the CD-ROM drive of FIG. 1.
A first gear 31 is installed at one end of the shaft of a sled motor 41. The first gear 31, a second gear 32, a third gear 33, a fourth gear 34, and a rack-shaped fifth gear 35 are sequentially engaged. Thus, the driving force from the sled motor 41 is sequentially transferred through the first gear 31, second gear 32, third gear 33, fourth gear 34, and fifth gear 35 to the pickup device 13, so that the pick up device 13 moves along the sled driving shaft 14.
A magnetic plate 42, on which a plurality of alternating north and south magnetic poles are formed as illustrated in FIG. 3, is installed on the shaft of the sled motor 41 between the first gear 31 and the sled motor 41. Also, a Hall-effect sensor 44 is installed at a predetermined position on a printed circuit board 43, separated by a predetermined distance from and opposed to the magnetic plate 42. Here, the combination of the magnetic disk 42 and the Hall-effect sensor 44 generates a signal whose frequency is proportional to the rotational speed of the sled motor 41.
FIG. 4 is a block diagram of a circuit for controlling the speed of a sled motor in a typical CD-ROM drive which uses a velocity profile to control the speed of the sled motor. The signal generated by the Hall-effect sensor 51 is amplified by an amplifier 52. A frequency-to-voltage converter 53 receives the amplified signal and converts the frequency of the amplified signal into a voltage to generate a converted voltage signal whose instantaneous value is proportional to the frequency of the received amplified signal. A microcomputer 55 generates a target velocity profile and outputs a target voltage, which represents a target speed, to be lowpass filtered in a lowpass filter (LPF) 56. Then, the converted voltage signal from the frequency-to-voltage converter 53 and the lowpass filtered signal are differentially amplified by being subtracted by a subtractor 57 and amplified by an amplifier and driver 58. Afterwards, the amplified signal drives a sled motor 59, so that the pick-up device moves in a desired manner.
When the CD-ROM drive is in an on-track state rather than a servo-control state, the converted voltage signal from the frequency-to-voltage converter 53 is differentially amplified with the lowpass filtered signal and a tracking error signal, which is output by a tracking error calculator 60 via a lowpass filter (LPF) 61, to drive the sled motor 59.
Thus, in the CD-ROM drive, the signal pulse generated by the Hall-effect sensor is converted into a voltage signal proportional to the frequency of the signal pulse and differentially amplified with a target speed voltage, so that the speed of the sled motor 41 is controlled to be close to the target speed.
Gains of the Hall-effect sensor 44 differ in each CD-ROM apparatus. Generally, the Hall gain depends on the magnetic field intensity of the magnetic plate 42, the sensitivity of the Hall-effect sensor 44, and. the distance between the magnetic plate 42 and the Hall-effect sensor 44, the most critical factor being the distance between the magnetic plate 42 and the Hall-effect sensor 44. If the distance is not adjusted precisely, which is very difficult, the output signal of the Hall effect sensor 44 which should reflect the actual rotating speed of the sled motor may not be accurate. Also, the inaccurate Hall gain due to the imprecise distance between the magnetic plate 43 and the Hall-effect sensor 44 results in an improper control of the sled servo.
If the Hall gain has an accurate or nominal value, the target velocity profile and the actual velocity profile have almost the same magnitude as shown in FIG. 5. However, if the Hall gain is inaccurate, the target velocity profile and the actual velocity profile have different magnitudes at each instant, as shown in FIGS. 6 and 7.
Specifically, if the Hall gain is greater than the nominal value, the output signal of the Hall effect sensor which is controlled to be the same as the target speed is greater than the actual speed, as shown in FIG. 6. Therefore, the actual rotating speed of the sled motor is smaller than the target speed. Contrarily, if the Hall gain is smaller than the nominal value, the output signal of the Hall effect sensor is smaller than the actual speed, as shown in FIG. 7. In such a case, the actual rotating speed of the sled motor is larger than the target speed.
Conventionally, to reduce the effect of the variation of the gain and regulate the output signal of the Hall-effect sensor 44, a variable resistor 54 is provided, as shown in FIG. 4, for adjusting the gain of the amplifier 52. However, the adjustment using the variable resistor 54 is not very accurate, and therefore a significant error remains.
To solve the above problems, it is an object of the present invention to provide a method for controlling the speed of a sled motor of an optical disk drive which can minimize the variation of the actual speed of the sled motor by detecting the actual speed thereof, and adaptively adjust the target speed according to the detected actual speed using a microcomputer.
To accomplish the above object, there is provided a method of controlling the speed of a sled motor for an optical disk wherein the sled motor is first driven according to a predetermined target velocity profile stored in a microcomputer. Then, a frequency of a sinusoidal wave output by a Hall-effect sensor is detected, and it is determined whether the frequency of the sinusoidal wave is higher than a predetermined value. If the frequency of the sinusoidal wave is greater than the predetermined value, the target speed is incremented by a predetermined amount. On the other hand, if the frequency of the sinusoidal wave is smaller than the predetermined value, the target speed is decremented by the predetermined amount.
In the sled motor speed controlling method according to the present invention, the variation of the gain of the Hall-effect sensor can be compensated during the operation of the optical disk drive. As a result, a faster access time can be obtained since the actual rotating speed of the sled motor is detected by the microcomputer and the target speed is adaptively adjusted in accordance with the actual detected speed.